Lithospheric scattering and structure beneath seismic arrays from
teleseismic P waveforms
Abstract
Random and small-scale subsurface heterogeneities in velocity and/or
density scatter the seismic wavefield when they have scale lengths on
the order of the seismic wavelength. Seismic scattering is considered
the origin of coda waves. Such inhomogeneities have an important effect
on propagating waves, as they generate traveltime and amplitude
fluctuations and may be the cause of attenuation or excitation of
secondary waves. Understanding the effect of small-scale heterogeneities
on the seismic wavefield is important for the characterization of the
seismic source (e.g. source parameters of underground nuclear
explosions) and to improve our knowledge of the Earth’s structure along
the raypath. Several approaches and methods have been suggested to study
the scattering of seismic waves and characterise subsurface
heterogeneities. Here, we apply a combination of the analysis of the
incoherent wavefield component and the coda decay with time to a dataset
of over 350 teleseismic events (over 20000 traces) recorded at three
seismic arrays (Warramunga, Alice Springs and Pilbara) in Australia.
This combination allow us to obtain a series of parameters (correlation
length, RMS velocity fluctuations of the heterogeneities and thickness
of the scattering layer) that give us a measure of the spatial scale and
the magnitude of the heterogeneities present in the lithosphere beneath
the arrays. This is the first time such a large dataset is used for a
study of these characteristics. Our new results show similar structures
and scattering strength for Alice Springs and Warramunga, while
revealing a different tectonic signature and stronger scattering in the
case of Pilbara, possibly caused by the different thicknesses of crust
and lithosphere between these regions and its different tectonic
history. These stochastic models of the lithosphere are the first step
in the development of a technique analogous to adaptive optics which, in
this case, aims at removing the effect of the small-scale, near receiver
structure from recorded wavefields, thus enabling us to improve our
source characterization and to more clearly image the Earth’s interior.